In gas turbine engines it is generally desirable for efficient operation to maintain minimum rotor tip clearances, and preferably with a substantially constant clearance around the circumference. This is the position for instance with cantilevered stators in an axial compressor or turbine. This can be difficult to achieve due for instance to various asymmetric effects either on build or during running. These effects include the casing centre being offset relative to the rotor drum centre line during build and/or during running. The casing may be distorted from a circular shape during build and/or running, and for instance the casing may become ovalised.